Using the generating functional method, the $\sigma$ and $\omega$ meson propagators in nuclear matter have been determined in a relativistic mean-field effective model where couplings between the nucleon and the mesons, quadratic in the meson fields, have been added to the standard Walecka Lagrangian. As an example of an application, the density dependence of the $\omega$ meson mass has been considered.

Fixed-$t$ subtracted dispersion relations are presented for Compton scattering off the nucleon at energies $E_{\gamma }<~500\hspace{0.5em}\mathrm{MeV},$ as a formalism to extract the nucleon polarizabilities with a minimum of model dependence. The subtracted dispersion integrals are mainly saturated by $\pi N$ intermediate states in the s channel, $\gamma \overrightarrow{N}\pi \overrightarrow{N}\gamma N,$ and $\pi \pi$ intermediate states in the t channel, $\gamma \overrightarrow{\gamma }\pi \overrightarrow{\pi }N\overline{N}.$ For the subprocess $\gamma \overrightarrow{\gamma }\pi \pi ,$ we construct a unitarized amplitude and find a good description of the available data. We show results for Compton scattering using the subtracted dispersion relations and display the sensitivity on the scalar polarizability difference $\alpha -\beta$ and the backward spin polarizability ${\gamma }_{\pi },$ which enter directly as fit parameters in the present formalism. Double polarization observables are shown to have a unique potential for measuring the spin polarizabilities of the nucleon.

The baryon masses are examined in $\mathrm{SU}\left(3\right)\mathrm{}$ chiral perturbation theory to third order using the recently proposed infrared regularization scheme. Fourth order is estimated by evaluating the dominant diagram. With this regularization the magnitude of the loop integrals is reduced so that the convergence of the series appears to be better than in the heavy baryon approach.

In a recent paper [Phys. Rev. C 57, 3406 (1998)] the performance of some quark potential models formulated in terms of effective interactions between constituent quarks has been studied. From this work the authors conclude that the quark models considered, including one-gluon exchange forces, fail in describing the baryon spectra. In this Comment we want to state precisely the working scheme followed in the above-mentioned paper to arrive at their conclusions, in order to show how the followed procedure makes some some of their statements invalid.

We emphasize the essential requirements for dealing with zero-range interactions in few-body systems. In particular, we make clear how the δ-function potentials appearing in nonrelativistic derivations of boson-exchange interactions are to be treated in order to provide sound input for quark model calculations of hadron spectra. The necessity of an exact solution of the few-quark system under consideration appears absolutely imperative. This condition is not met by some hybrid quark models we have tested in our original paper. As a result they fail in describing the N and Δ spectra.

The microscopic structure of the spin-dipole resonance (SDR) at $E_x=21.1\mathrm{}\hspace{0.5em}\mathrm{MeV}$ in ${}^{208}\mathrm{Bi}$ has been investigated in the ${}^{208}\mathrm{Pb}{(}^3\mathrm{He}{,t)}^{208}\mathrm{Bi}$ reaction at ${E(}^3\mathrm{He})=450\mathrm{}\hspace{0.5em}\mathrm{MeV}$ and very forward scattering angles. Protons emitted due to the decay of the SDR were measured in solid-state detectors in coincidence with tritons detected with the high resolution magnetic spectrometer Grand Raiden. The total and partial branching ratios for proton decay from the SDR to the residual neutron-hole states in ${}^{207}\mathrm{Pb}$ have been measured. The deduced escape widths are found to be in good agreement with recent theoretical estimates.

The nuclei ${}^{138}\mathrm{Nd}$ and ${}^{139}\mathrm{Nd}$ have been studied at very high spins via the ${}^{48}\mathrm{Ca}{+}^{94}\mathrm{Zr}$ reaction. Several new rotational bands were observed, four in ${}^{138}\mathrm{Nd}$ and two in ${}^{139}\mathrm{Nd}.$ The $J^{\mathrm{}\left(2\right)\mathrm{}}$ moments of inertia calculated from the observed $\gamma$-ray energies are very small and almost constant, indicating that these bands are triaxial. Cranked Nilsson-Strutinsky calculations reproduce the general behavior of the bands, supporting this interpretation and suggesting an approximately constant $\gamma$ value of $\sim +35\mathrm{}°$ over a large spin range up to the highest observed spins. These bands and a few similar bands in other nuclei of the $N\approx 80$ region are a unique example of almost undisturbed triaxial bands.

New band structures in the neutron-rich ${}^{113,115,117}\mathrm{Pd}$ nuclei have been constructed by measuring the $\gamma -\gamma -\gamma$ coincidences emitted from the spontaneous fission of ${}^{252}\mathrm{Cf}$ with GAMMASPHERE. The triple $\gamma$-ray coincidence data revealed new negative-parity bands built on ${11/2}^{-}$ isomeric states in ${}^{113,115,117}\mathrm{Pd}.$ These bands built on the $\nu h_{11/2}$ orbital suggest a band crossing at a frequency of $ħ\omega \approx 0.45\hspace{0.5em}\mathrm{MeV}.$ The rotational behavior and energy level spacings of these bands suggest a prolate shape. Two previously unreported bands are also observed in ${}^{113,115}\mathrm{Pd}$ and tentatively assigned positive parity. These have band crossings at 0.25 and 0.32 MeV.

The cross sections of the ${(e,e}^{\prime }N)$ and $(\gamma ,p)$ reactions on ${}^{16}\mathrm{O}$ are calculated, for the transitions to the ${1/2}^{-}$ ground state and the first ${3/2}^{-}$ excited state of the residual nucleus, using single-particle overlap functions obtained on the basis of one-body density matrices within different correlation methods. The electron-induced one-nucleon knockout reaction is treated within a nonrelativistic distorted wave impulse approximation framework. The theoretical treatment of the $(\gamma ,p)$ reaction includes both contributions of the direct knockout mechanism and of meson-exchange currents. The results are sensitive to details of the different overlap functions. The consistent analysis of the reaction cross sections and the comparison with the experimental data make it possible to study the nucleon-nucleon correlation effects.

Based on the Zimányi-Moszkowski derivative coupling Lagrangian plus tensor-coupling term, calculations of symmetric semi-infinite nuclear matter and finite nuclei have been performed. Our results show that with increasing tensor coupling the spin-orbit potential is considerably improved, but the surface tension and the surface thickness decrease to values too low compared with experiment. In addition, the effects of the $\sigma$-meson mass on nuclear surface properties, on the spin-orbit potential as well as on nuclear binding energies are discussed. No parameter set could be found that leads to good surface properties even if energies and spin-orbit splittings are realistic.

We compute the recoil polarization of the lambda hyperon and the photon asymmetry for the quasifree photoproduction of kaons in a relativistic mean-field approximation to the Walecka model. Our motivation for studying polarization observables is threefold. First, polarization observables are more effective discriminators of subtle dynamics than the unpolarized cross section. Second, earlier nonrelativistic calculations suggest that polarization observables are relatively insensitive to distortions effects. Finally, this insensitivity entails an enormous simplification in the theoretical treatment. Indeed, by introducing the notion of a “bound-nucleon propagator” we exploit Feynman’s trace techniques to develop closed-form, analytic expressions for all photoproduction observables. Moreover, our results indicate that polarization observables are also insensitive to relativistic effects and to the nuclear target. Yet, they are sensitive to the model parameters, making them ideal tools for the study of modifications to the elementary amplitude—such as in the production, propagation, and decay of nucleon resonances—in the nuclear medium.

The recent preliminary values of ${A(Q}^2)$ and $t_{20}\left(70\mathrm{}°\right)$ elastic electron-scattering data extracted from deuterium at larger values of $Q^2$ have placed more stringent constraints on models of the nucleon-nucleon system. Comparisons of the data with conventional hadronic models and with perturbative QCD have been made, and are strongly in favor of the hadronic picture provided isoscalar meson exchange currents are included. In this work a nonperturbative quark model of the deuteron with parameters determined from earlier work is used to investigate the role of quark correlations in describing the deuteron electromagnetic form factors. The results give a good description of the recent data and show that this model with many-body confinement has quark correlation contributions to the charge form factors which are remarkably similar to the meson exchange current contributions.

Extraction of spin observables from vector meson photoproduction on a nucleon target is described. Starting from density matrix elements in the vector meson’s rest frame, we transform to spin observables in the photon-nucleon c.m. frame. Several constraints on the transformed density matrix and on the spin observables follow from requiring that the angular distribution and the density matrix be positive definite. A set of constraints that are required in order to extract meaningful spin observables from forthcoming data are enunciated.

Polarized proton beams of 80 and 60 keV, and a biased target have been used to determine the slope of the astrophysical S factor for the ${}^7\mathrm{Li}(p,\gamma {)}^8\mathrm{Be}$ reaction from direct measurements at proton energies from 40 to 100 keV for capture to both the ground and the first excited states of ${}^8\mathrm{Be}.$ The results indicate a negativeslope for both cases. Vector analyzing power measurements at $90°$ were also obtained for proton energies from 40 to 80 keV. It is shown that the negative slope can be accounted for by considering the effect of the subthreshold resonance state at 16.6 MeV if the capture into this weakly bound, subthreshold state occurs at an effective radius of about 46 fm. The effect of this state increases the extrapolated value of the S factor at $E_p=0\mathrm{}$ from the previous value of 0.3 keV b to the present value of 0.50 keV b.

We calculate neutrino reaction rates with nucleons via the neutral and charged currents in the supernova core in the relativistic random phase approximation (RPA) and study their effects on the opacity of the supernova core. The formulation is based on the Lagrangian employed in the calculation of the nuclear equation of state (EOS) in the relativistic mean field theory (RMF). The nonlinear meson terms are treated appropriately so that the consistency of the density correlation derived in RPA with the thermodynamic derivative obtained from the EOS by the RMF is satisfied in the static and long wavelength limit. We employ pion and rho meson exchange interactions together with the phenomenological Landau-Migdal parameters for the isospin-dependent nuclear interactions. We find that both the charged and neutral current reaction rates are suppressed from Bruenn’s standard approximate formula considerably in the high density regime $({\rho }_b\gtrsim {10}^{14}{\mathrm{g}/\mathrm{c}\mathrm{m}}^3$ with ${\rho }_b$ the baryonic density). In the low density regime $({\rho }_b\lesssim {10}^{14}{\mathrm{g}/\mathrm{c}\mathrm{m}}^3),$ on the other hand, the vector current contribution to the neutrino-nucleon scattering rate is enhanced in the vicinity of the boundary of the liquid-gas phase transition, while the other contributions are moderately suppressed there also. In the high temperature regime $(T\gtrsim 40\mathrm{MeV}$ with T the temperature) or in the regime where electrons have a large chemical potential, the latter of which is important only for the electron capture process and its inverse process, the recoil of nucleons cannot be neglected and further reduces the reaction rates with respect to the standard approximate formula which discards any energy transfer in the processes. These issues could have a great impact on the neutrino heating mechanism of collapse-driven supernovae.

High-spin states were populated in ${}^{108}\mathrm{Cd}$ using the ${}^{96}\mathrm{Zr}{(}^{16}\mathrm{O},4n)$ reaction at a beam energy of 72 MeV. Two magnetic dipole bands have been observed, both of which contain weak $E2\mathrm{}$ crossover transitions. Lifetimes for the stronger of the two bands were measured via the Doppler shift attenuation method. The configuration assignment for this band has been determined from comparison with tilted axis cranking model calculations to be $\pi \left[g_{9/2}^{-3}{g}_{7/2}\right]\otimes \nu \left[h_{11/2}{(g}_{7/2}{d}_{5/2}{)}^1\right]$ and $\pi \left[g_{9/2}^{-3}{g}_{7/2}\right]\otimes \nu \left[h_{11/2}^3{(g}_{7/2}{d}_{5/2}{)}^1\right],$ before and after the $\nu h_{11/2}^2$ band crossing, respectively. The deduced $B\left(M1\mathrm{}\right)\mathrm{}$ strengths decrease rapidly with increasing spin after the $\nu {\mathrm{h}}_{11/2}^2$ alignment, which is a characteristic behavior of a shears band.

The strength function of a single state $|d〉$ is studied using the deformed Gaussian orthogonal ensemble. In particular we study the dependence of the spreading width of $|d〉$ on the degree of mixing.

Using the GAMMASPHERE array in conjunction with the Fragment Mass Analyzer, the level structure of the near drip-line nucleus ${}^{178}\mathrm{Hg}$ has been considerably expanded with the recoil-decay tagging technique. Of particular interest is a new rotational band which exhibits a complex decay towards the low spin states arising from both the prolate-deformed and the nearly spherical coexisting minima. It is proposed that this band is associated at low spin with an octupole vibration which is crossed at moderate frequency by a shape driving, two quasiproton excitation.

We suggest using polarized nuclear targets of ${}^7\mathrm{Li}$ and ${}^3\mathrm{He}$ to study nuclear effects in the spin-dependent structure functions $g_{1A}{(x,Q}^2).\mathrm{}$ These effects are expected to be enhanced by a factor of 2 as compared to the unpolarized targets. We predict a significant x dependence at ${10}^{-4}–{10}^{-3}<~x<~0.2$ of $g_{1A}{(x,Q}^2{)/g}_{1N}{(x,Q}^2)$ due to nuclear shadowing and nuclear enhancement. The effect of nuclear shadowing at $x\approx {10}^{-3}$ is of an order of 16% for $g_{1A=7}^{n.s.\mathrm{}3/2}{(x,Q}^2{)/g}_{1N}^{n.s.\mathrm{}}{(x,Q}^2)$ and 10% for $g_{1A=3}^{n.s}{(x,Q}^2{)/g}_{1N}^{n.s.\mathrm{}}{(x,Q}^2).\mathrm{}$ By imposing the requirement that the Bjorken sum rule is satisfied we model the effect of enhancement. We find the effect of enhancement at $x\approx 0.125\hspace{0.5em}\mathrm{}\left(0.15\right)\mathrm{}$ to be of an order of $20\hspace{0.5em}\mathrm{}\left(55\right)\%$ for $g_{1A=7}^{n.s.\mathrm{}3/2}{(x,Q}^2{)/g}_{1N}^{n.s.\mathrm{}}{(x,Q}^2)$ and $14\hspace{0.5em}\mathrm{}\left(40\right)\%$ for $g_{1A=3}^{n.s}{(x,Q}^2{)/g}_{1N}^{n.s.\mathrm{}}{(x,Q}^2),$ if enhancement occupies the region $0.05<~x<~0.2\hspace{0.5em}(0.1\mathrm{}<~x<~0.2).$ We predict a 2% effect in the difference of the scattering cross sections of deep inelastic scattering of an unpolarized projectile off ${}^7\mathrm{Li}$ with $M_J=3/2\mathrm{}$ and $M_J=1/2.\mathrm{}$ We also show explicitly that the many-nucleon description of deep inelastic scattering off ${}^7\mathrm{Li}$ becomes invalid in the enhancement region $0.05<x<~\mathrm{0.2.}$

We calculate in (two-loop) chiral perturbation theory the local $\mathrm{NN}$ potentials generated by certain classes of three-pion exchange diagrams. We stress that the chiral $3\pi \mathrm{NN}$-contact vertex depends on the particular choice of the interpolating pion-field and, therefore, one has to consider representation-invariant classes of diagrams by supplementing graphs involving the chiral $4\pi$ vertex. We find that the resulting isovector spin-spin and tensor $\mathrm{NN}$ potentials are negligibly small for $r>~0.8$ fm. One can conclude that the effects of uncorrelated chiral $3\pi$ exchange in the $\mathrm{NN}$ interaction are very small and therefore of no practical relevance.